# 7: Entropy and the Second Law

- Page ID
- 202920

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- 7.1: A System and Its Surroundings
- A primary goal of the study of thermochemistry is to determine the quantity of heat exchanged between a system and its surroundings. The system is the part of the universe being studied, while the surroundings are the rest of the universe that interacts with the system. A system and its surroundings can be as large as the rain forests in South America or as small as the contents of a beaker in a chemistry laboratory.

- 7.2: The Second Law of Thermodynamics
- An isolated system is a little more than just adiabatic. In the latter heat cannot get in or out. In an isolated system nothing gets in or out, neither heat nor mass nor even any radiation like light. The isolated system is like a little universe all to itself

- 7.3: We Must Always Devise a Reversible Process to Calculate Entropy Changes
- The second law of thermodynamics can be formulated in many ways, but in one way or another they are all related to the fact that there is a state function S that at least in isolated systems tends to increase. The second law has important consequences for the question of how we can use heat to do useful work.

- 7.5: Entropy Can Be Expressed in Terms of a Partition Function
- We have seen that the partition function of a system gives us the key to calculate thermodynamic functions like energy or pressure as a moment of the energy distribution. We can extend this formulism to calculate the entropy of a system once its Q is known. The derivation is shown on page 840 and involves the use of the Stirling approximation. The end result is